Firearms and Toolmarks. Firearms Summary Guns (types and manufacturing) Ammunition Firing a gun Collecting Evidence Analyzing Evidence.
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Firearms and Toolmarks
Firearms Summary
Guns (types and manufacturing) Ammunition Firing a gun Collecting Evidence Analyzing Evidence
Types of Guns
Firearms
Smooth Barrel: Hollow tube
Firearms
Types of Barrels
Rifled Barrel: Barrel of gun can be rifled to put spin on bullet; increases accuracy
Rifled Barrel
Inside of barrel has hills (lands) and valleys (grooves)
Lands
Grooves
Lands and grooves twist down length of gun
Firearms
Rifled Barrel
Inside of barrel has hills (lands) and valleys (grooves)
Lands and grooves twist down length of gun
Firearms
Rifled Barrel (manufacturing)
Begin with a hollow tube Lands and grooves made by forcing cutting
tools through the tube.
Firearms
Rifled Barrel (manufacturing)
The minor differences in manufacturing leads to differences in the scratches (striations) on the fired bullet.
Firearms
Handgun
Designed to be held and fired with one hand
Revolver Semiautomatic pistol
Firearms
Revolver
Hammer pulled back manually or when trigger is pulled
Hammer hits back of bullet casing; causes explosion
Hammer Bullets
Trigger
Barrel rifled
Firearms
Semiautomatic pistol
Bullet casing automatically ejected.
Hammer Bullets
Trigger
Hammer strikes bullet case when trigger pulled
Barrel rifled
Firearms
Rifle
Hammer strikes bullet case when trigger pulled
Bullet casing manually or automatically ejected.
Bullets
Trigger
Bullets
Hammer
Barrel rifled; fires bullet
Firearms
Shotgun
Hammer strikes shotshell when trigger pulled
Bullet casing manually or automatically ejected.
Projectile
Trigger
Barrel smooth; fires shot or slug
Firearms
Ammunition
(Bullets)
Bullet
Cartridge
Primer
Propellant (inside cartridge)
Firearms > Ammunition
Ammunition (Types of Bullets)
1. Lead or Lead Alloy: softer bullet; scraped up by the lands and grooves of barrel
2. Semijacketed bullet: brass jacket covers sides of lead bullet, except for tip
Bullet expands on impact Jacket not scraped up as much Jacket can be shed after hitting target
Firearms > Ammunition
Ammunition (Types of Bullets)
3. Full Metal Jacket: lead core completely covered with brass
Firearms > Ammunition
Shotgun Shell
Shot (pellets)
Wad (plastic)
Cartridge
Primer
Propellant (inside cartridge)
Firearms > Ammunition
Fires many small round pellets
Primer
When hit with hammer, primer creates a small explosion
This explosion lights propellant creating a large explosion (propels bullet)
Firearms > Ammunition
Propellant
Modern propellants are smokeless powders composed of nitrocellulose
Some contain nitrocellulose and nitroglycerin
Firearms > Ammunition
Collection of Firearms evidence
Important marks on the outside of the bullet can be damaged during collection.
Either collect the item containing embedded bullet or…
Use rubber coated forceps to remove bullet
Firearms > Collection
Fired Bullets
Firearms > Collection
Collection of Firearms evidence
Bullets and cartridges should probably not be physically marked by investigator. Marks may damage important striations
Small evidence should be collected in sealed and marked containers Pillbox, glass vials
Firearms > Collection
Analysis of Firearms evidence
Class Characteristics of bullets:
Caliber (size) # of lands/grooves Direction of rifling
twist
Degree of twist Width of
lands/grooves
Firearms > Analysis
Analysis of Firearms evidence
Class Characteristics of cartridges
Caliber (size) Firing pin location Firing pin size/shape
Extractor/ejector sizes Relative location of
extractor/ejector
Firearms > Analysis
Analysis of Firearms evidence
If class characteristics of fired bullet and suspected gun match, a test bullet is fired for further analysis.
Firearms > Analysis
Comparison microscope is used to compare striations on bullets.
Firearms > Analysis
Comparison of Striations
Striations – tiny scratches on bullet that can be specific to a single gun.
Firearms > Analysis
Conclusions of analysis
If class characteristics and striations match, can say the bullet was likely fired by the suspect gun.
If class characteristics (and striations) do not match, the suspect gun can be excluded.
If class characteristics match but striations do not, the results are inconclusive.
Firearms > Analysis
Range-of-fire estimation
The distance between the gun and victim can be estimated by looking at gunshot residue patterns
Long distance: no GSR on the victim
Close-range: GSR on victim - spread out
Near-contact: GSR on victim - concentrated circle
Contact shot: “Blown out” entrance wound
Firearms
Tool MarksTools make specific marks when they cut or
scrape a softer surface
Compression tool mark: shows outline of tool (hammer into wood)
Sliding tool mark: parallel striations when tool slides across material (screwdriver or crowbar)
Cutting tool mark: striations when tool cuts through material (scissors)
Tool Marks
Collection of Tool Marks
Notes, sketches, photographs
If possible, the tool mark should be taken back to the lab.
If not, a cast should be made.Silicone rubber ideal material
Tool Marks
Conclusions of analysis
If class characteristics and striations match, can say the tool mark was likely made by the suspect.
If class characteristics (and striations) do not match, the suspect tool can be excluded.
If class characteristics match but striations do not, the results are inconclusive.
Firearms > Analysis
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